Bandwidth+ is a Mac program that lives on your Menu Bar and provides you with details regarding your network usage. The application is simple to handle, provides you with a comprehensive help manual, and offers you accurate information.
Once you launch the utility on your Mac, a new icon will be displayed on your status bar. By clicking it, you will reveal the network usage info. You can choose which type of information to be displayed on the main menu of the program. The settings can be found on the Preferences panel of the application.
An advantage is that the tool makes and keeps a log with all of the fetched details.
The only disadvantage I was able to find while testing this program was that it supports only one output format (CSV) for exporting history logs. Mac OS X doesn't provide you with a default tool for opening this type of files and you need to download and install a compatible utility from the web to read the generated reports.
Still, Bandwidth+ provides you with a simple and quick solution for monitoring your network usage. Not to mention, it comes without a price tag. So, you have nothing to lose if you decide to give it a try on your Mac.